As described in German patent document 3,530,812 a thermostatic mixing valve intended for domestic bathroom use has a housing formed with separate hot- and cold-water input connections and with a tempered-water output connection typically carrying a faucet. A valve body in the housing can move between full-cold and full-hot end positions to admit hot and cold water in different ratios, depending on the desired setting which itself is established by an adjustment knob or lever. A thermostat, which term here means a device capable of changing position or shape dependent on temperature, can act on the valve body to move it so as to keep the temperature of the water downstream of the valve body constant. Normally this thermostat is braced by a return spring with the valve body against the temperature-setting device.
In such an arrangement this return spring, therefore, is downstream of the valve body. Thus water flows over and through it whenever the valve is in use. The result is, of course, that this part can lime up, corrode, or otherwise become nonoperational.
Furthermore, although modular valve construction is known from German patent document 3,239,772, in such thermostatic mixing valves any repairs normally entail substantial work with the actual valve housing itself. As a result servicing such a valve is fairly difficult.